In recent years, there have been a variety of requirements, such as improved fuel efficiency and compliance with emission regulations for internal-combustion engines. In response to these requirements, in diesel-engine vehicles, a method of improving fuel efficiency by increasing the supercharging pressure of a turbocharger, improving engine output ratio and thereby achieving a reduction in engine size have been widely adopted. Further, in order to comply with emission regulations, a low-pressure loop (LPL)-EGR system for increasing the amount of exhaust gas recirculation (EGR) gas has been increasingly adopted.
In a compressor of a turbocharger equipped with an LPL-EGR system, the compressor outlet temperature is increased when the supercharging pressure of the turbocharger is increased, and soot-containing deposit originated from an engine oil are formed in the compressor (hereinafter, such deposits are referred to as “compressor deposits”). Since this deposit formation reduces the turbocharger efficiency, the output temperature must be controlled in order to prevent the formation of such deposits. Accordingly, increasing the output temperature by inhibiting the deposit formation has been studied. Non-patent Literature 1 describes that the evaporation characteristics of an engine oil affects the deposit formation and the deposit formation can be suppressed by limiting the amount of light fraction in the oil. Patent Literature 1 describes that sludge formation in the turbo mechanism of an engine equipped with a direct-injection turbo mechanism is inhibited by reducing the amount of light fraction of a lubricant composition.
Patent Literature 2 discloses a lubricant composition which is used for reducing the total hydrocarbon emissions from a diesel engine and comprises a Fischer-Tropsch-derived base oil and at least one additive. Patent Literature 3 discloses a lubricant composition which provides improved fuel efficiency characteristics while maintaining desirable wear performance and NOACK volatility, and discloses that when a Fischer-Tropsch-derived base oil is not used, volatility control is not lost. However, neither Patent Literature 2 nor Patent Literature 3 describes deposit reduction focusing on the distillation characteristics of the Fischer-Tropsch-derived base oil.
Patent Literature 4 discloses a lubricant composition for attaining lubricity and heat resistance at high temperatures in a turbocharger lubricant, which lubricant composition comprises a combination of base oils each having a specific kinematic viscosity and additives. However, Patent Literature 4 does not describe that deposits are attributed to the distillation characteristics of the base oils.